It is sought more particularly here below in this document to describe problems existing in the field of marine navigation for seismic prospection. The invention of course is not limited to this particular field of application but is of interest for any sail path determining method and device that has to cope with closely related or similar issues and problems.
A marine seismic acquisition in a survey area conventionally use networks of sensors distributed along cables in order to form linear acoustic antennas, also referred to as “streamers” or “seismic streamers”. The seismic streamers are towed through water behind a vessel or a fleet of vessels at a variable water depth.
A seismic method is usually based on analysis of reflected seismic waves. Thus, to collect geophysical data in the marine environment, one or more submerged seismic sources (e.g. air guns) are activated in order to propagate seismic wave trains. The pressure wave generated by the seismic source passes through the column of water and propagates through the different layers of the sea bed, part of the acoustic waves reflecting on the layers interfaces. Reflected signals are then detected by the seismic sensors distributed over the length of the seismic streamers, digitized and transmitted to a central unit onboard the vessel, where they are stored and processed.
A key driver of a marine seismic survey sits in the minimization of the survey operational expenditures, which partially rely on the time spent surveying. Consequently, time saving is of prime importance in a marine seismic survey.
In operation, as shown in FIG. 1, the survey is discretized into sail paths 12, 13, 14 . . . , 17 along which the vessel 10 must sail. A sail line is an arbitrary geometric curve, which can be a straight line, a curve line, or a combination of both for example. Each theoretical location where a seismic source must shoot, is a shot point location (also referred to as “shot point”) SP defined by its geographical coordinates. When the seismic source reaches this shot point, it is activated to produce an acoustic wave. The shot points SP of seismic sources are arranged along the sail paths, also called “preplot” lines.
The acquisition process is controlled and monitored by a navigation system (also referred to as INS, for “Integrated Navigation System”), which is onboard each vessel and whose role is to compute position of sources and sensors if any, to drive the vessels along their sail path according to the predefined preplot geometry, and to activate sources to perform seismic acquisition at desired location. In case a fleet of vessels forms a cluster operating the seismic survey, also referred to as a multi-vessel operation, one vessel is set as a master of the cluster. So does its navigation system. A multi-vessel operation requires the vessels to match a predetermined geometry, relatively to their master, when the fleet is surveying a preplot line. This induces severe timing constraints on the position of each vessel relatively to the master, as they will start shooting a preplot line. The vessel pattern must be properly set-up when arriving to the start of preplot line.
To carry out a global coverage of the survey area, each vessel involved in the survey needs to turn from a preplot line to another preplot of the survey area. For example, vessel 10 will turn from a point A, also called end of line (EOL) point, which constitutes the end of the preplot line 12, to a point B, also called start of line (SOL) point, which constitutes the start of the preplot line 17. Each vessel has its own turn radius. This physical constraint is notably due to the length of towed streamers 18, which can be up to several kilometers long.
In the following description, an arc of circle designates an unbroken portion of the circumference of a circle or any other curved line.
The sail path 20 the vessel 10 shall use to perform its turn can be composed of an arc of a start circle CA passing through the EOL point A with a radius greater than or equal to the vessel's turn radius, an arc of an end circle CB passing through the SOL point B with a radius greater than or equal to the vessel's turn radius, and a straight segment linking the arcs of the start and end circles.
A problem arises when an obstacle 11 is located on the sail path that the vessel 10 shall initially use to perform its turn (such as oil platform, rig, wells, FPSO (“Floating Production Storage Offloading”) unit, etc.). In that case, a new sail path shall be determined so as to get around this obstacle or those obstacles during the vessel's turn.
This operation of determination of a new sail path (which avoids the obstacles) is manually done by a human operator onboard the vessel, by using the information given by a navigation software. The operator typically watches a navigation screen on which is edited the sail path that the vessel shall use during the turn from the start point A to the end point B, to verify that no collision with obstacles will occur. If an obstacle is detected as being likely to interfere with the vessel on the sail path originally defined by the software, the operator has at his disposal a graphical tool that allows to graphically add an extra turn, wherever he desires, based on a circle for each detected obstacle to get around them. The extra circle or circles graphically added are then taken into account by the software to compute a new sail path from the start point A to the end point B so as to adjust the vessel course accordingly. The software determines the vessel's sail path using combinations of some basic trigonometric calculations taking into account tangential points. In practice, the sail path is constantly adjusted visually by the operator.
However, this well-known solution involves a human operator at each stage of the process, which is not optimal. Indeed, the graphic adjustment of the sail path by the operator is inherently rough, thereby resulting in suboptimal sail path computation and operational expenditures. In addition, the responsibility of the safety with regards to obstacle avoidance is totally deported to the operator.
In multi-vessel survey, this problem is made more complex since the operator has to determine an optimal sail path for each of the vessels involved in the survey, while ensuring that the vessels do not collide one with another or with fixed or moving obstacles.
Another difficulty for the operator is to obtain a perfect synchronization of all vessels, possibly moving at different speeds, so that they end their turn at the same time.
Doing so manually by taking all these constraints into account is very difficult to achieve even for an experienced user. This is all the more true since the number of vessels to control and the number of obstacles present in the navigation area is important.